Address compiler warnings on 6.1.4, closes #402

Of the warnings listed in #402 (closed):

• containments/homescreens/halcyon/plugin/applicationlistmodel.cpp has a [-Wunused-parameter] addressed here ( d5afe7f0 )
• quicksettings/screenshot/screenshotutil.cpp has a [-Wdeprecated] addressed here (93dc1b3b). TBH I'm not an experienced developer so I haven't understood what this is about. I'm following what is recommended in http://eel.is/c++draft/depr#capture.this. When you're not comfortable, we can drop this commit.
• containments/homescreens/halcyon/plugin/pinnedmodel.cpp has a [-Wunused-parameter] addressed here (d5afe7f0)
• kwin/mobiletaskswitcher/effecttouchborder.cpp has a [-Wunused-parameter] addressed here (d5afe7f0)
• components/shellsettingsplugin/mobileshellsettings.cpp has a [-Wunused-parameter] addressed here (d5afe7f0)
• kcms/cellularnetwork/modemdetails.cpp has a [-Wswitch] due to cases introduced in ModemManager 1.20. I've taken the description of the new cases from ModemManager-enums.h (LL 149-150, 220-221). Addressed here (5c3298a9). About MM_MODEM_ACCESS_TECHNOLOGY_LTE_CAT_M and MM_MODEM_ACCESS_TECHNOLOGY_LTE_NB_IOT, they look like low-power and low-bitrate LTE modes: being LTE, I've gone with 4G description instead of previous generations with lower specs, to avoid complications.
• kcms/cellularnetwork/mobileproviders.cpp has [-Wdeprecated-declarations] related to QLocale. To be discussed if the variables' name in the same code area are to be s/country/territory/ as well... Addressed here (2d5026ac).
• containments/homescreens/folio/delegatetoucharea.cpp has [-Wunused-parameter] addressed here (d5afe7f0)
• components/wallpaperimageplugin/wallpaperplugin.cpp has a [-Wunused-parameter] addressed here (d5afe7f0)
• components/mobileshell/components/swipearea.cpp has [-Wunused-parameter] that were already addressed by @devinlin with 3adcd1d5